1. Field of the Invention:
The invention concerns self-extinguishing thermoplastic molding compounds (compositions) which contain polyphenylene ethers as their main component.
2. Discussion of the Background:
Polyphenylene ethers are polymers with good thermal and electrical properties. Poly(2,6-dimethyl,1,4-phenylene ether) (PPE) has achieved technical significance.
Pure PPE is difficult to process due to its high melt viscosity, but results in molded parts with high shape retention under heat. However, they are not very impact-resistant, especially in the areas having external notches.
There have been numerous attempts to improve the processing properties and notch impact resistance of the molding masses (see DE-PS 16 94 255 corresponding to U.S. Pat. No. 3,361,851, DE-PS 16 94 257 corresponding to U.S. Pat. No. 3,383,435 and DE-PS 16 95 290 corresponding to U.S. Pat. No. 3,379,792). However, it has been shown that adding polyolefins, polystyrenes and polyamides does not influence the property profile of molding materials containing PPE in an optimal way.
Mixtures of polyphenylene ethers with impact-resistant polystyrenes have achieved greater technical significance (see DE-PS 21 19 301 and DE-PS 22 11 005). Since the addition of polystyrene improves the melt flow, these mixtures can be easily processed to yield molded parts with sufficient impact resistance, but have the disadvantage that with an increasing polystyrene content, the flammability increases, and at the same time, the shape retention under heat decreases.
In order to reduce the flammability of the molding compounds containing PPE, various flame retardant agents are used. Suitable substances have proven to be aromatic phosphorus compounds such as triphenyl phosphate, diphenyl cresyl phosphate and/or diphenyl isopropylphenyl phosphate and others, for example, which also serve as a processing aid. Processing at higher temperatures, however, causes unpleasant odors, and harmful physiological effects of the aromatic phosphorus compounds or their decomposition products, respectively, cannot be eliminated.
It is known that halogen containing substances can be used as flame retardant agents. Possible substances are, for example, organic compounds such as those described in the monograph by H. Vogel, "Flammfestmachen von Kunststoff", Huethig-Verlag, 1966, on pages 94 to 102. Halogenated polymers, such as halogenated polyphenylene ethers, for example (e g DE-PS 33 34 068), especially brominated polyphenylene ethers or brominated oligostyrenes or polystyrenes with preferably more than 30 % by weight halogen are used as flame retardants. If flame retardant agents which contain halogens are used, it is recommended that compounds of antimony, boron or tin be added in amounts from 0.5 to 10 % by weight, relative to the thermoplastic resin, in order to increase the effect of flame retardancy.
A serious disadvantage of all organic compounds which contain halogens is that highly corrosive and toxic hydrogen chloride or hydrogen bromide gas is formed during fires.
DE-OS 29 06 336 describes the use of special hydrocarbons as synergists for organic halogen compounds, especially bromine compounds, for providing flame protection of expandable suspension polystyrene. Suitable hydrocarbons are, e.g., benzyl toluene and dibenzyl toluene, also in the form of the isomer mixtures. In addition to 0.1-3 % by weight of the hydrocarbon, halogen compounds are used in such an amount that the halogen content is 0.1-2 % by weight relative to the styrene polymer. In a preferred embodiment, 0.01-0.08 % by weight of organic peroxides are present, in addition, which have a half life period of more than 2 hours at 100.degree. C. Such molding compounds are only suitable for the production of foam polystyrene with flame retardant properties and demonstrate the disadvantages stated above, because of the unavoidable halogen content.